In digital communication systems and digital broadcasting systems, a signal is transmitted as an electromagnetic waveform through a physical channel, such as air, to receivers. A channel effect may occur that is non-ideal, including multipath reflection and propagation fading, that leads to signal distortion. When a receiver receives a multipath signal resulting from such multipath reflection with a large delay spread, which is equivalent to a narrow coherent bandwidth, the coherent bandwidth of the multipath signal is smaller than that of a single path signal and the channel response of the signal results in frequency selective fading. In the channel with selective fading, an OFDM based wireless communication system may overcome this kind of channel response.
OFDM techniques may be utilized in wireless communication systems and in digital video and audio broadcasting systems for high spectrum efficiency transmission. In OFDM systems, two types of network construction are multiplied frequency networks (MFN) and single frequency networks (SFN). A single frequency network is a broadcast network in which several transmitters simultaneously send the same signals over the same frequency channels. Advantages of SFNs include: (1) wide coverage of the network with a large number of transmitters and small transmission power; (2) good frequency efficiency with one frequency band for all transmitters in the network; and (3) mobile users can receive signals in the SFN and do not need to switch to another frequency band when entering an adjacent transmitting area. Systems that utilize SFNs may include, but are not limited to, Digital Video Broadcasting-Terrestrial (DVB-T), Digital Audio Broadcasting-Handheld (DVB-H), Digital Audio Broadcasting (DAB), Terrestrial Digital Multimedia Television Broadcasting (DMB-T), and Media-FLO.
In addition to decreasing the effect of multipath selective fading, channel encoding and time interleaving functions of an OFDM system can also enhance system performance and correct error bits. For better bit correcting ability, channel encoding and time interleaving functions can be used in conjunction with diversity techniques for improving channel response. Three diversity techniques are used in many systems, including time diversity, frequency diversity, and space diversity. Channel response with better diversity provides correct signal bits for channel decoding to correct the incorrect bits of the remaining channel responses. Transmitting diversity or receiving diversity is applied to OFDM wireless communication systems to provide greater channel variety and to allow good diversity gain to enhance system performance.
SFNs for OFDM wireless communication systems allow for wide signal coverage and a large number of transmitters. However, since receivers can instantaneously receive the same signals from two or more transmitters from the same SFN, the system performance when receivers are at the boundaries of coverage of two or more transmitters can be degraded. If a delay spread between the two or more signals is small, which is equivalent to wideband coherent bandwidth, and the variance of the channel response is slow, which can result in a flat fading channel, the receivers may receive a signal resulting from destructive interference that is caused by the two or more signals having different phase rotations and arriving at the same time. If a long coherent time of the signal caused by a shadowing effect arises with a low flat channel response, the system performance can degrade over a long period of time. Such system degradation may occur even though the OFDM system decreases the effect of selective channel fading and there are channel encoding and time interleaving techniques preventing consecutive errors and correcting error bits of the remaining channel responses.
For example, FIGS. 1A and 1B illustrate a conventional OFDM wireless communication system 100 with flat fading channel response caused by a receiver 110 receiving two signals at coverage boundaries of a transmitter 102 of area-A and a transmitter 108 of area-B, both of which transmit the same signals 112 and 118 containing pilot signals p and data signals d(0), d(1), . . . , d(k), respectively. With reference also to FIG. 1B, the signal 112 of OFDM symbols from the transmitter 102 of area-A passes through channel A with channel response ha 104. The signal 118 of OFDM symbols from the transmitter 108 of area-B passes through channel B with channel response hb 106. If a received signal 120 from the transmitters 102 and 108 is s01 then:s01=s00*(ha+hb),where s00 is the signal 112 from the transmitter 102 or the signal 118 from the transmitter 108. A multipath delay between the two signals 112 and 118 from area-A and area-B, respectively, may be small and a destructive interference may be generated because of different phase rotations in the channel responses. When the channel response ha is similar to −hb, the combined effect of the channel responses, i.e., ha+hb, may be a flat and wideband fading channel response on the receiver side. In addition, because a low received signal magnitude 122 caused by the destructive interference may be smaller than a threshold value 124 of the signal detector in the receiver 110, detection of the received signal 120 may fail and the channel decoder of the receiver 110 may also fail to correct error bits.
Diversity techniques can help prevent the low and flat channel responses. FIG. 2 shows a method for enabling a mobile station 250 to obtain a sector diversity gain by applying space-time coding (STC) to the signals transmitted in each sector in order to improve reception performance of the mobile station 250 in a cell/sector boundary in accordance with U.S. Pub. No. 2005/0265280. Each OFDM transmitter of base stations 2101-210M includes an STC encoder 211, a selection controller 230, and a selector 213, and can produce a diversity gain at cell/section boundaries. The selection controller 230 controls the selector 213 to form properly encoded symbols after the STC encoder 211. In each sector, space-time code streams to be transmitted in the sector are selectively output according to a corresponding one of the control signals, forming a specific per-cell pattern, and the pattern is changed for each update interval of the space-time code stream, in order to provide uniform sector diversity to the mobile station 250.
FIG. 3 shows a method for transmitting a broadcasting channel by means of cyclic delay diversity in an OFDM mobile communication system in accordance with U.S. Pub. No. 2005/0281240. In this method, each transmitter is controlled by a cyclic delay controller 370 to make different cyclic delays with the same transmitted data and hence a mobile station receives signals with diversity gain.
FIG. 4 illustrates a group scrambling method to provide channel diversity of multi-cell Multimedia Broadcast Multicast Service (MBMS) transmission in accordance with “R1-061264: Further study on reference signal structure for MBMS,” 3GPP LTE RAN1 meeting document by Toshiba Corp., NTT DoCoMo. According to this method, subcarriers on which MBMS data are transmitted can be divided into several subcarrier groups, e.g., groups 411-413 for a first transmitter and groups 421-423 for a second transmitter. All data and pilot signals of each subcarrier group are multiplied by a normalized random complex number from a scrambling pattern 414 or 424. At coverage boundaries of the two transmitters, a receiver receives a signal 430 from the two transmitters with scrambling of the random complex numbers in each subcarrier group. Scrambling of the complex random number in each subcarrier group is intended to prevent destructive signal interference. Channel response 431 for each subcarrier group is of low correlation and hence a frequency diversity effect is obtained. However, this group scrambling method may be unattractive for channel estimation in frequency domain because of non-continuity in channel response, which may result in decrease of channel estimation accuracy and increase of channel estimation complexity on the receiver side. In addition, the slow fading channel of a subcarrier group by a shadowing effect may keep an almost unchanged low channel response for all time in such diversity scenarios. That part of the signal may not be correctly detected.